Therapy with radiolabeled antibodies has been introduced against non-Hodgkin lymphoma (NHL) and is an approved method today. Two products are on the market, Zevalin™ and Bexxar™, and both targets the CD20 antigen (Jacene et al., 2007).
Also the immunotherapeutic agent rituximab (Rituxan™/Mabthera™) targets the CD20 antigen. One problem with treatment against the same target is the possibility of immunophenotypic drift during the disease course (Ngo et al., 2009) which could cause diminished effects of CD20 therapy when repeated over time as in rituximab therapy or if CD20-based radioimmunotherapy (RIT) is administered following prolonged rituximab therapy.
A large number of patients receiving CD20 directed therapy will eventually experience relapse (Buchegger et al., 2006; Gordon et al 2004). Thus, there is a significant need for RIT that targets another antigen than the CD20 in NHL patients.
In the early development of RIT, the two antigens CD37 and CD20 were evaluated as targets (Press et al., 2001). It was concluded that the CD20 targeting RIT was more appropriate and therefore the development of CD37 directed RIT was abandoned. Thus, it is known in the art that monoclonal antibodies are suitable for use in RIT against lymphoma, but that radioimmunoconjugate (RIC) targeting CD20 is superior to RIC targeting CD37 (Press et al., 2001).
In recent years CD37 has attracted some new interest (Heider et al., 2009; Grosmaire, 2007), mainly as target for immunotherapy using chimeric or humanized antibody constructs. These works teaches away from using conventional murine IgG monoclonal antibodies, since murine antibodies may induce human anti-mouse antibody (HAMA) production in patients, which can cause discomfort and reduced efficacy of immunotherapies.
For RIT, conventional murine monoclonal antibodies are still considered interesting, since in general the protein doses used are lower and the treatment need not being repeated to the same extent as with immunotherapy. Also the clearance of murine IgG is generally slightly faster than humanized or chimeric versions of the same IgG, which may be more appropriate in terms of whole body radiation exposure from RIT, at least in some settings. It should be noted that both Bexxar and Zevalin are based on murine antibodies.
The present invention provides the anti-CD37 murine antibody HH1 as carrier for radioisotope. The original hybridoma clone that produces the murine anti-CD37 antibody HH1 was developed in the 1980's (Smeland et al., 1985) and the HH1 antibody has been in sale for in vitro use in immunohistochemistry for several years.
HH1 has not previously been evaluated for radioimmunotherapy in terms of biodistribution and cellular cytotoxicity. The current work was therefore undertaken to evaluate the suitability of HH1 in radioimmunotherapy. In contrast to the previous clinical and preclinical work with anti-CD37 RIC, which used 131I directly radiolabeled to the tyrosine residues using the chloramineT/Iodogen methods, the HH1 was radiolabeled via a chelator using a metallic radionuclide instead of a halogen.
Using a metallic radionuclide labeled via a chelator-linker could be advantageous since the use of 131I-labeled antibodies is associated with the exposure of the thyroid to various amounts of iodine released from the RIC's.
In a previous study to evaluate whether HH1 was suitable for producing a radioimmunoconjugate CHX-A-DTPA was conjugated to HH1 and the conjugate labeled with 205,206Bi for in vitro modeling purposes (Henriksen et al., 1997). The uptake in the cell line Raji was compared for bismuth conjugated to HH1 or streptavidin. In the latter case cells had been presaturated with biotinylated-HH1.
It was found that the number of chelators required to ensure functional RIC when labeled with 212Bi or 213Bi was a limiting factor. It was therefore suggested to use biotinylated HH1 instead of a HH1 based RIC. Once bound to the cells, the biotinylated HH1 could then be targeted with radiolabeled streptavidin.
Thus, the work suggests that HH1 labeled with an alpha-particle-emitting radionuclide was less useful due to insufficient specific activity at the chelator concentrations deemed tolerable for the HH1 to retain sufficient binding ability.
It was also indicated in the paper that a beta-emitter would be even less suitable for constructing a functional RIC compared with an alpha-emitter (Henriksen et al, 1997) as the authors stated that targeted radiotherapy with beta-emitter should be inferior in disseminated disease because cross-fire is essential for obtaining sufficient effect.
Thus, the above cited work teaches away from using a directly chelated HH1 in radioimmunotherapy and also away from using HH1 in a beta-emitter based RIC.